Cell Biology

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1. Cell Structure

1.1 Plant and Animal Cells

All living organisms are made of cells. This is the first principle of cell theory, proposed by Schleiden and Schwann in the 1830s and extended by Virchow in 1855. Animal and plant cells share Many structures but have key differences. Understanding these differences is fundamental: they Explain why plants can stand upright without a skeleton, why only plant cells can photosynthesise, And why plant cells have a fixed, rectangular shape while animal cells are flexible and irregular.

Structure	Function	Plant/Animal
Cell membrane	Controls what enters and leaves the cell; partially permeable	Both
Cytoplasm	Gel-like substance where chemical reactions occur	Both
Nucleus	Contains genetic material (DNA); controls cell activities	Both
Mitochondria	Site of aerobic respiration; produces ATP (energy)	Both
Ribosomes	Site of protein synthesis	Both
Cell wall	Made of cellulose; provides structural support	Plant only
Permanent vacuole	Contains cell sap; helps maintain cell shape	Plant only
Chloroplasts	Site of photosynthesis; contain chlorophyll	Plant only (some cells)

The cell membrane as a gatekeeper. The cell membrane is not a solid barrier; it is a dynamic Structure composed of a phospholipid bilayer with embedded proteins. Small, non-polar molecules (such as $\mathrm{O_2$ and $\mathrm{CO_2$ ) can diffuse directly through the lipid bilayer. Larger Or polar molecules (such as glucose and ions) require transport proteins to cross. This selective Permeability is essential for maintaining the internal conditions of the cell (homeostasis).

The nucleus as the control centre. The nucleus contains the cell’s DNA, which carries the Instructions for making every protein the cell needs. DNA is organised into structures called Chromosomes, which become visible as distinct threads during cell division. When a cell needs to Make a particular protein, the relevant section of DNA (the gene) is “read” and a copy is made in The form of messenger RNA (mRNA). The mRNA leaves the nucleus through nuclear pores and travels to The ribosomes, where the protein is assembled.

Mitochondria as the powerhouses. Mitochondria are the sites of aerobic respiration, where Glucose is broken down using oxygen to release energy in the form of ATP. Cells that have high Energy demands (muscle cells, liver cells, sperm cells) contain large numbers of mitochondria. Mitochondria have a double membrane: the outer membrane is smooth, while the inner membrane is Folded into projections called cristae. The cristae increase the surface area available for the Chemical reactions of respiration, increasing the rate of ATP production.

Ribosomes as the protein factories. Ribosomes are the sites of protein synthesis. They read the MRNA code and assemble amino acids in the correct order to make proteins. Ribosomes are found free In the cytoplasm (making proteins for use within the cell) and attached to the rough endoplasmic Reticulum (making proteins for export or insertion into the cell membrane).

Chloroplasts as the solar panels. Chloroplasts are the sites of photosynthesis in plant cells. They contain the green pigment chlorophyll, which absorbs light energy (mainly red and blue Wavelengths; green light is reflected, which is why plants appear green). Chloroplasts have a double Membrane and contain internal membranes called thylakoids, which are stacked into grana. The Light-dependent reactions of photosynthesis occur on the thylakoid membranes, while the Light-independent reactions (Calvin cycle) occur in the stroma (the fluid surrounding the Thylakoids). Not all plant cells contain chloroplasts — root cells, for example, are Underground where there is no light.

The cell wall as structural support. The plant cell wall is made of cellulose, a tough, fibrous Polysaccharide. It provides mechanical strength and rigidity, allowing plant cells to maintain a Fixed shape. Unlike the cell membrane, the cell wall is fully permeable to small molecules. The cell Wall also prevents plant cells from bursting when they take in water by osmosis.

The permanent vacuole as a hydrostatic skeleton. The permanent vacuole is a large, central Compartment filled with cell sap (a solution of sugars, salts, and pigments). It pushes the Cytoplasm against the cell wall, creating turgor pressure. This pressure keeps the cell firm and Provides structural support to the plant. When a plant is well-watered, all its cells are turgid and The plant stands upright. When the plant is dehydrated, the cells lose water, become flaccid, and The plant wilts.

1.2 Eukaryotic and Prokaryotic Cells

Eukaryotic cells (animals, plants, fungi) have a membrane-bound nucleus and membrane-bound Organelles (mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus). The word “eukaryotic” literally means “true kernel,” referring to the presence of a true nucleus. The DNA in Eukaryotic cells is linear and organised into multiple chromosomes.

Prokaryotic cells (bacteria and archaea) are smaller and simpler:

No membrane-bound nucleus (circular DNA is free in the cytoplasm, in a region called the nucleoid)
No membrane-bound organelles (no mitochondria, no chloroplasts, no ER, no Golgi)
Plasmids (small, extra rings of DNA that can carry genes for antibiotic resistance)
A cell wall (made of peptidoglycan, not cellulose)
Flagellum (tail for movement, in some bacteria)
Capsule (slime layer for protection against the host immune system, in some bacteria)
Pili (short hair-like projections for attachment or conjugation, in some bacteria)

Why the size difference matters. Prokaryotic cells are 1—10 micrometres in diameter, While eukaryotic cells are 10—100 micrometres. The smaller size of prokaryotes means they have a Larger surface-area-to-volume ratio, which is sufficient because they lack internal organelles and Rely on diffusion for transport of materials. Eukaryotes, being larger, need internal compartments (organelles) to compartmentalise biochemical reactions and maintain locally high concentrations of Reactants.

A practical consequence of this size difference is that prokaryotes can reproduce much faster than Eukaryotes. Under ideal conditions, some bacteria can divide every 20 minutes, whereas eukaryotic Cells take hours to divide. This rapid reproduction is why bacteria can quickly develop Resistance to antibiotics and why bacterial infections can spread so rapidly.

Detailed comparison:

Feature	Eukaryotic cells	Prokaryotic cells
Nucleus	Yes, membrane-bound	No (nucleoid region only)
DNA	Linear, in chromosomes	Circular, single
Membrane-bound organelles	Yes (mitochondria, ER, Golgi, etc.)	No
Ribosomes	80S (larger)	70S (smaller)
Cell wall	Cellulose (plants), chitin (fungi)	Peptidoglycan
Size	10—100 $\mu$ M	1—10 $\mu$ M
Reproduction	Mitosis, meiosis	Binary fission
Plasmids	No	Yes
Examples	Animals, plants, fungi, protists	Bacteria, archaea

1.3 Cell Specialisation

Cells are specialised to perform specific functions. Every specialisation you encounter can be Understood by asking one question: what structural feature increases the rate of whatever process The cell needs to carry out?

Cell Type	Specialisation	Function
Sperm cell	Long tail, many mitochondria, acrosome	Fertilise egg
Nerve cell	Long axon, branched dendrites	Transmit electrical impulses
Muscle cell	Contain many mitochondria, can contract	Movement
Root hair cell	Elongated projection, large surface area	Absorb water and minerals
Red blood cell	Biconcave disc, no nucleus, contains haemoglobin	Transport oxygen
White blood cell	Can change shape, contains enzymes	Defend against pathogens
Palisade cell	Many chloroplasts, tall and thin	Photosynthesis
Egg cell	Large, contains food store, cytoplasm	Fertilisation
Ciliated cell	Has cilia that beat to move mucus	Remove debris from airways

Building intuition for specialisation. The red blood cell is a particularly elegant example. Its Biconcave disc shape maximises surface area for oxygen diffusion. It has no nucleus — this might Seem like a disadvantage, but it creates more space inside the cell for haemoglobin, the protein That carries oxygen. The trade-off is that red blood cells cannot divide or repair themselves; they Have a limited lifespan of about 120 days and are continuously replaced in bone marrow. At any given Moment, you have approximately 25 trillion red blood cells circulating in your body, and Approximately 2.4 million new ones are produced every second.

The sperm cell illustrates another principle: energy supply. The many mitochondria in the midpiece Provide the ATP needed for the tail (flagellum) to beat and propel the sperm towards the egg. The Acrosome at the tip contains digestive enzymes that break down the outer layers of the egg, allowing The sperm to penetrate. Only a few hundred sperm out of millions will reach the egg, and normally Only one will fertilise it.

The nerve cell (neurone) is adapted for rapid, long-distance communication. Its long axon (which can Be up to 1 metre in the sciatic nerve) allows electrical impulses to travel quickly over long Distances. The myelin sheath (an insulating layer of fat) further speeds up transmission by allowing The impulse to “jump” between gaps in the myelin (nodes of Ranvier). Branched dendrites at the Receiving end connect to many other neurones, allowing complex networks of communication.

The root hair cell is an excellent example of how surface area relates to function. The elongated Hair-like projection massively increases the surface area in contact with the soil water, allowing More efficient absorption of water and mineral ions. The root hair cell also has many mitochondria To supply ATP for the active transport of mineral ions from the soil (where their concentration is Often very low) into the cell.

The egg cell (ovum) is the largest human cell, with a diameter of approximately 0.1 mm (visible to The naked eye). It contains a large cytoplasm with stores of nutrients (lipid droplets and protein Granules) to sustain the early embryo before it implants in the uterus and establishes a blood Supply from the mother.

1.4 Required Practical: Microscopy

Method:

Clip a thin section of onion epidermis onto a slide.
Add a drop of iodine solution (stains the cell and makes nuclei visible).
Place a cover slip at an angle to avoid air bubbles.
Start with the lowest magnification objective lens.
Focus using the coarse then fine adjustment.
Increase magnification as needed.

Magnification calculation:

$\mathrm{Magnification = \frac{\mathrm{image size}{\mathrm{actual size}$

This equation can be rearranged to find any of the three variables:

$\mathrm{Actual size = \frac{\mathrm{image size}{\mathrm{magnification}$

$\mathrm{Image size = \mathrm{actual size \times \mathrm{magnification$

A useful mnemonic: the MIA triangle (Magnification = Image / Actual).

Worked Example. A cell is observed to be 4.2 mm in a micrograph. The actual size is 0.07 mm. Calculate the magnification.

$\mathrm{Magnification = \frac{4.2}{0.07} = 60$

Unit conversion tip. You will frequently need to convert between millimetres (mm) and Micrometres ( $\mu$ M). Remember: $1 \mathrm{ mm = 1000 \mathrm{ \mu m$ .

Worked Example. A cell measures 8.5 mm in a micrograph taken at magnification $\times 400$ . Find The actual size in micrometres.

$\mathrm{Actual size = \frac{8.5}{400} = 0.02125 \mathrm{ mm = 21.25 \mathrm{ \mu m$

Staining. Most biological specimens are colourless and transparent, making them difficult to see Under a microscope. Stains such as iodine (which stains starch and nuclei), methylene blue (which Stains nuclei and DNA), and eosin (which stains cytoplasm) add colour and contrast. Different stains Bind to different cellular components, allowing specific structures to be identified.

Safety precautions when using a microscope:

Always carry the microscope with two hands (one on the arm, one supporting the base).
Start with the lowest magnification objective lens to find the specimen before increasing magnification.
Never use the coarse adjustment knob when looking through the high-power objective — this can crack the slide and damage the lens.
When preparing a slide, always lower the cover slip at an angle to avoid trapping air bubbles.

2. Cell Division

2.1 Mitosis

Mitosis is cell division that produces two genetically identical daughter cells, each with the Same number of chromosomes as the parent cell. It is used for growth, repair, and asexual Reproduction.

The cell cycle:

The cell cycle is not just mitosis. In fact, the majority of the cell’s life is spent in interphase, not in division. Understanding this distinction is critical because many exam Questions test whether you know that DNA replication occurs during interphase, not during mitosis.

Interphase: The cell grows and replicates its DNA. Each chromosome is copied to form two identical chromatids joined at the centromere. Interphase is subdivided into:

G1 (Gap 1): Cell grows and carries out normal functions. The duration of G1 varies enormously between cell types. Some cells (neurones) never leave G1 and do not divide again.
S (Synthesis): DNA is replicated. Each chromosome now consists of two sister chromatids, which are exact copies of each other.
G2 (Gap 2): Cell prepares for division; organelles are replicated, proteins for spindle fibres are synthesised, and the cell checks that DNA replication was completed accurately.

Mitosis: The cell divides. The stages are:

Prophase: Chromosomes condense and become visible as distinct structures (under a microscope, they appear as X-shaped because each chromosome consists of two chromatids). The nuclear membrane begins to break down. Spindle fibres begin to form from the centrioles (in animal cells).
Metaphase: Chromosomes line up at the centre of the cell (the metaphase plate), attached to spindle fibres at their centromeres. This alignment ensures that each daughter cell receives exactly one copy of each chromosome.
Anaphase: Spindle fibres contract, pulling chromatids apart to opposite poles. The chromatids are now individual chromosomes. The cell visibly elongates.
Telophase: New nuclear membranes form around each set of chromosomes. Chromosomes decondense (become thin and thread-like again). The spindle fibres break down.

Cytokinesis: The cytoplasm divides, producing two daughter cells. In animal cells, the cell membrane pinches inwards (cleavage furrow) to divide the cell. In plant cells, a cell plate forms down the middle, which develops into a new cell wall.

Why chromosomes must line up at the metaphase plate. This is a quality control step. If Chromosomes did not line up correctly, one daughter cell might receive two copies of a chromosome While the other receives none. This would be lethal or cause serious genetic disorders. The cell has Checkpoint mechanisms that delay anaphase until all chromosomes are correctly attached to spindle Fibres.

2.2 The Importance of Mitosis

Growth: Multicellular organisms grow by increasing the number of cells. A single fertilised egg undergoes repeated mitotic divisions to form a complete organism containing trillions of cells. In humans, the process from a single cell to birth involves approximately 41 rounds of cell division (though not all cells divide this many times).
Repair: Damaged or dead cells are replaced. Skin cells are constantly replaced by mitosis in the basal layer of the epidermis (the outer layer of skin is shed and replaced every few weeks). When you cut your finger, mitosis generates new cells to heal the wound. Red blood cells are replaced at a rate of approximately 2.4 million per second.
Asexual reproduction: Produces genetically identical offspring (clones). Bacteria reproduce asexually by binary fission (a form of mitosis). Strawberry plants produce runners (horizontal stems) that develop into genetically identical plants. Potato tubers are underground stems from which new genetically identical plants grow.

2.3 Stem Cells

Stem cells are unspecialised cells that can differentiate into specialised cell types. They are The biological equivalent of blank templates: they carry the complete set of genetic instructions But have not yet activated the specific subset of genes that defines a specialised cell.

How differentiation works. Every cell in your body contains the same DNA (with minor exceptions Such as red blood cells, which have no DNA at all). The difference between a nerve cell and a muscle Cell is not the DNA but which genes are switched on (expressed) and which are switched off. During Differentiation, certain genes are permanently switched off. Once a cell has differentiated, it cannot change back (this is called being “terminally differentiated”).

Type	Source	Potential
Embryonic stem cells	Early embryos	Can become any cell type (totipotent/pluripotent)
Adult stem cells	Bone marrow, brain, etc.	Can become a limited range of cell types
Plant stem cells	Meristems (tips of shoots and roots)	Can become any plant cell type

Totipotent vs. Pluripotent vs. Multipotent. A totipotent stem cell can become any cell type, Including placental cells. Only the zygote and the first few cells of the early embryo (up to the 4-cell stage in humans) are totipotent. A pluripotent stem cell can become any cell type except Placental cells. Embryonic stem cells (from the blastocyst stage) are pluripotent. A multipotent Stem cell can become a limited range of cell types within a particular tissue. Adult stem cells (e.g., those in bone marrow) are multipotent.

Uses of stem cells:

Treating leukaemia (bone marrow transplants — adult stem cells from a donor replace the patient’s cancerous blood cells)
Research into regenerative medicine (growing new tissues to replace damaged organs; clinical trials are underway for growing heart valve tissue, retinal cells for treating blindness, and nerve cells for treating spinal cord injuries)
Drug testing (testing pharmaceutical compounds on differentiated human cells grown from stem cells, reducing the need for animal testing and providing more relevant results since human cells respond differently to drugs than animal cells)
Treating spinal cord injuries and degenerative diseases such as Parkinson’s disease (where dopamine-producing nerve cells in the brain die; stem cells could potentially replace them)

Ethical considerations:

Embryonic stem cell research involves the destruction of embryos, at the blastocyst stage (around 5—7 days after fertilisation). Opponents argue that this embryo has the potential to become a person and therefore has a right to life.
Adult stem cells are less controversial but more limited in potential. They can only differentiate into a restricted range of cell types.
Some religious and philosophical perspectives hold that human life begins at conception; others argue that an early embryo, which has no nervous system and cannot feel pain, does not have the moral status of a person.
In many countries, embryonic stem cell research is regulated, with restrictions on the sources of embryos and the types of experiments permitted.
In the UK, embryonic stem cell research is permitted under licence from the Human Fertilisation and Embryology Authority (HFEA), using embryos donated with informed consent from IVF treatment.
Therapeutic cloning: creating an embryo by somatic cell nuclear transfer (the same technique used to create Dolly the sheep) to produce stem cells that are genetically identical to the patient, avoiding immune rejection. This is ethically controversial because it involves creating and destroying an embryo.

2.4 Cancer

Cancer is the result of uncontrolled cell division. It occurs when mutations in genes that control The cell cycle (specifically proto-oncogenes, which promote cell division, and tumour Suppressor genes, which inhibit it) cause cells to divide repeatedly without stopping, forming a tumour.

Benign tumours: Grow slowly, remain in one place, not dangerous. They are contained within a fibrous capsule and do not spread to other parts of the body. They can still cause problems if they press on organs or blood vessels (e.g., a benign brain tumour can cause headaches and vision problems by pressing on surrounding tissue).
Malignant tumours: Grow rapidly, can spread to other parts of the body (metastasis), life-threatening. Cancer cells lose their normal adhesion properties, break away from the primary tumour, and travel through the bloodstream or lymphatic system to form secondary tumours (metastases) in other organs. Metastasis is what makes malignant cancer so dangerous: it is not the primary tumour that kills the patient, but the secondary tumours that disrupt the function of vital organs.

Risk factors:

Smoking: introduces carcinogens (cancer-causing chemicals) into the lungs. Tar in tobacco smoke damages DNA in lung cells, leading to mutations. Smoking is responsible for approximately 87% of lung cancer deaths.
UV radiation: causes mutations in skin cells, particularly damaging DNA by forming thymine dimers (abnormal bonds between adjacent thymine bases). This is the primary cause of skin cancer (melanoma). Sunburns, especially in childhood, significantly increase the risk.
Ionising radiation (X-rays, gamma rays): can directly damage DNA, causing double-strand breaks.
Some viruses: HPV (human papillomavirus) is linked to cervical cancer; hepatitis B and C are linked to liver cancer. These viruses can insert their DNA into the host cell’s genome, disrupting tumour suppressor genes or activating proto-oncogenes.
Genetic predisposition: inherited mutations in tumour suppressor genes (e.g., BRCA1 and BRCA2) increase the risk of breast and ovarian cancer. People with these mutations do not inevitably develop cancer, but their risk is significantly higher than average.

3. Transport Across Cell Membranes

3.1 Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of Lower concentration, down the concentration gradient.

Why diffusion works (the physical intuition). Particles are in constant random motion due to Their kinetic energy. In a region of high concentration, there are more particles moving in any Given direction. In a region of low concentration, there are fewer. The net result is that more Particles move from high to low concentration than the reverse, until equilibrium is reached (equal Concentration on both sides). At equilibrium, particles continue to move, but there is no net Movement in either direction.

Key features:

Passive process (no energy required from the cell)
Occurs in both liquids and gases
Rate increases with: larger concentration gradient, higher temperature, larger surface area, shorter diffusion distance
Only works for small, non-polar molecules (e.g., $\mathrm{O_2$ , $\mathrm{CO_2$ ) and lipid-soluble substances
Large or polar molecules (e.g., glucose, amino acids, ions) cannot diffuse through the lipid bilayer and require transport proteins (facilitated diffusion or active transport)

Factors affecting the rate of diffusion:

Factor	Effect on Rate	Why
Concentration gradient	Increases rate	Larger difference means more net movement
Temperature	Increases rate	Particles have more kinetic energy, move faster
Surface area	Increases rate	More area available for particles to cross
Diffusion distance	Decreases rate	Longer distance means slower transfer
Molecular size	Smaller molecules diffuse faster	Less resistance; larger molecules collide more frequently
Membrane permeability	More permeable = faster diffusion	Determines which substances can cross freely

Worked Example. Oxygen diffuses from the alveoli (high concentration) into the blood (low Concentration). Carbon dioxide diffuses from the blood into the alveoli. This simultaneous exchange Of gases is efficient because both processes use the same concentration gradient in opposite Directions: the alveoli have high $\mathrm{O_2$ and low $\mathrm{CO_2$ (because fresh air is Continually breathed in), while the blood arriving at the lungs has low $\mathrm{O_2$ and high $\mathrm{CO_2$ (because it has just returned from the body tissues where respiration occurred).

3.2 Osmosis

Osmosis is the net movement of water molecules across a partially permeable membrane from a Region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution).

Water potential ( $\psi$ ) is a measure of the tendency of water to move from one area to another. Pure water has the highest water potential (defined as 0). Adding solutes lowers the water potential (makes it negative). The more solute that is dissolved, the lower (more negative) the water Potential.

Key terms:

Hypotonic: Lower solute concentration than the cell (higher water potential; water enters the cell)
Isotonic: Equal solute concentration (equal water potential; no net movement of water)
Hypertonic: Higher solute concentration than the cell (lower water potential; water leaves the cell)

What happens to cells in different solutions:

Solution	Animal cell	Plant cell
Hypotonic	Swells and may burst (lysis)	Becomes turgid (firm, rigid)
Isotonic	Normal	Normal (slightly flaccid)
Hypertonic	Shrinks (crenation)	Plasmolyses (membrane pulls away from cell wall)

Why plant cells do not burst. Plant cells have a rigid cell wall made of cellulose. When water Enters by osmosis in a hypotonic solution, the cell expands but the cell wall exerts an inward Pressure (turgor pressure) that prevents further expansion. This turgor pressure is essential for Maintaining the structure of non-woody plants — without it, they wilt. Animal cells lack a cell Wall, so they continue to expand until they burst (lyse). This is why hospitals use saline drips (isotonic solution) rather than pure water for intravenous injections: pure water would cause red Blood cells to burst.

Plasmolysis in plant cells. In a hypertonic solution, water leaves the plant cell by osmosis. The cell membrane pulls away from the cell wall, and the cell becomes flaccid. Unlike animal cells, Plant cells do not completely collapse because the rigid cell wall maintains the cell’s shape. Plasmolysis can be observed under a microscope when onion epidermal cells are placed in a Concentrated sugar solution.

3.3 Required Practical: Osmosis in Potato Chips

Method:

Use a cork borer to cut potato cylinders of identical diameter.
Trim them to the same length using a ruler and scalpel.
Measure and record the initial mass of each chip using a balance.
Place each chip in a beaker with a different concentration of sugar solution (e.g. 0%, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm $^3$ ).
Leave for a set time (e.g. 30 minutes) at the same temperature.
Remove, gently dry with a paper towel, and measure the new mass.
Calculate the percentage change in mass:

$\mathrm{Percentage change = \frac{\mathrm{final mass - \mathrm{initial mass}{\mathrm{initial mass} \times 100\%$

Plot a graph of percentage change in mass against sugar concentration.

Expected results:

At 0% (distilled water): mass increases (water enters by osmosis; the solution is hypotonic).
At high concentration (e.g. 1.0 mol/dm $^3$ ): mass decreases (water leaves by osmosis; the solution is hypertonic).
At a specific concentration (e.g. 0.3 mol/dm $^3$ ): no change (the solution is isotonic; the water potential inside and outside the potato cells is equal). This point can be read from the graph where the line crosses the x-axis.

Variables:

Independent variable: concentration of sugar solution.
Dependent variable: percentage change in mass of the potato chip.
Control variables: volume of solution, temperature, time left in solution, surface area of chip, type and age of potato.

3.4 Active Transport

Active transport is the movement of particles against the concentration gradient (from low to High concentration). It requires energy from respiration (ATP) and carrier proteins (pumps) in the Membrane.

Why active transport is necessary. Diffusion alone cannot always supply a cell with everything It needs. For example, root hair cells in plants need to absorb mineral ions from the soil, where Their concentration is already very low. Diffusion would move ions out of the cell, not into it. Active transport uses energy to pump ions into the cell against the concentration gradient.

Example: Mineral ions in the soil are absorbed by root hair cells against the concentration Gradient. The root hair cells have many mitochondria to supply the ATP needed for active transport. This is why root hair cells in low-nutrient soils can still absorb minerals: they are not limited by The concentration gradient in the soil.

Another example: In the small intestine, glucose is absorbed from the gut into the blood. After A meal, the concentration of glucose in the gut may be higher than in the blood, so diffusion Accounts for some absorption. However, when most of the glucose has been absorbed and the Concentration in the gut drops below that in the blood, diffusion would stop. Active transport Continues to pump glucose into the blood against the concentration gradient, ensuring that all Available glucose is absorbed.

Comparison of transport mechanisms:

Feature	Diffusion	Osmosis	Active Transport
Direction	High to low concentration	High to low water potential	Low to high concentration
Energy required	No	No	Yes (ATP)
Membrane needed?	No	Yes (partially permeable)	Yes (carrier proteins)
What moves?	Any particles	Water molecules only	Specific molecules/ions
Can it reach eqm?	Yes	Yes	No (works to create gradient)

4. Cell Organisation

4.1 Levels of Organisation

$\mathrm{Cells \to \mathrm{Tissues \to \mathrm{Organs \to \mathrm{Organ Systems \to \mathrm{Organisms$

Cell: The basic unit of life.
Tissue: A group of similar cells with the same function (e.g., muscle tissue is made of muscle cells; epithelial tissue is made of epithelial cells).
Organ: A group of tissues working together to perform a specific function (e.g., the heart is made of muscle tissue, connective tissue, nervous tissue, and blood).
Organ system: A group of organs working together (e.g., the circulatory system includes the heart, blood vessels, and blood).
Organism: A living thing made up of various organ systems.

4.2 Examples in the Human Body

Level	Example
Cell	Muscle cell
Tissue	Muscle tissue
Organ	Heart
Organ system	Circulatory system
Organism	Human

Another example — the digestive system:

Level	Example
Cell	Epithelial cell
Tissue	Epithelial tissue
Organ	Small intestine
Organ system	Digestive system
Organism	Human

5. Higher Tier: Microscopy and Cell Size Calculations

5.1 Electron Microscopy

Light microscopes are limited by the wavelength of visible light (approximately 400—700 nm). The Maximum resolution of a light microscope is about 200 nm, which means it cannot distinguish two Objects closer together than 200 nm. Most organelles are larger than this, but many internal Structures (ribosomes, plasma membrane, endoplasmic reticulum) are below this limit.

Electron microscopes use a beam of electrons instead of light. Electrons have a much shorter Wavelength, allowing resolution down to about 0.5 nm (Transmission EM) or about 5 nm (Scanning EM). This is sufficient to see individual molecules and large proteins.

Transmission EM (TEM): Electrons pass through a thin specimen. Produces a 2D image showing Internal ultrastructure. Specimens must be cut into very thin sections (ultrathin sections) using an Ultramicrotome. The image appears in shades of grey (dense structures appear darker because they Scatter more electrons).

Scanning EM (SEM): Electrons bounce off the surface of the specimen. Produces a 3D image of the Surface. The specimen is coated with a thin layer of metal (e.g., gold) to make it conductive.

Limitations of electron microscopy:

Specimens must be placed in a vacuum, so only dead specimens can be viewed.
Specimens must be chemically fixed and dehydrated, which may introduce artefacts (structures that do not exist in living cells).
Preparation is complex and time-consuming.
The images are black and white (though they can be artificially coloured using computer software).

5.2 Standard Form and Unit Conversions

Biological measurements often involve very small numbers. You must be confident converting between Units.

Unit	Symbol	Conversion to metres
Millimetre	mm	$\times 10^{-3}$ m
Micrometre	$\mu$ M	$\times 10^{-6}$ m
Nanometre	nm	$\times 10^{-9}$ m

Worked Example. A mitochondrion is $5 \mu\mathrm{m$ long. Express this in nanometres.

$5 \mathrm{ \mu m = 5 \times 1000 \mathrm{ nm = 5000 \mathrm{ nm$

5.3 Estimating Cell Size

To estimate the size of a cell or organelle from a micrograph:

Measure the diameter of the field of view at a known magnification.
Estimate how many cells fit across the diameter.
Divide the field of view diameter by the number of cells.

Worked Example. A student observes a cell under a microscope at $\times 400$ magnification. The Diameter of the field of view at this magnification is $0.5 \mathrm{ mm$ . The student estimates That 10 cells fit across the diameter. Calculate the width of one cell in micrometres.

$\mathrm{Width of one cell = \frac{0.5 \mathrm{ mm}{10} = 0.05 \mathrm{ mm = 50 \mathrm{ \mu m$

Common Pitfalls

Confusing mitosis and meiosis. Mitosis produces 2 identical daughter cells (growth/repair); meiosis produces 4 non-identical gametes (reproduction). Mitosis maintains the diploid chromosome number; meiosis halves it. Crossing over only occurs in meiosis (Prophase I).
Forgetting that osmosis refers to water movement only. Other substances move by diffusion or active transport. If a question asks about the movement of glucose, the answer is diffusion or active transport, never osmosis.
Confusing eukaryotic and prokaryotic cells. Bacteria are prokaryotic (no nucleus, no membrane-bound organelles, circular DNA, plasmids, peptidoglycan cell wall). Everything else (animals, plants, fungi) is eukaryotic.
Writing the magnification equation the wrong way round. Magnification = image size / actual size. A common mistake is to write actual / image, which gives a tiny number rather than the magnification.
Stating that diffusion requires energy. Diffusion is passive; only active transport requires energy. If you write “diffusion requires ATP” you will lose marks.
Confusing the terms tissue, organ, and organ system. Make sure you can give specific examples at each level.
Describing the cell cycle as just mitosis. Interphase is the longest phase of the cell cycle. DNA replication occurs during interphase (S phase), not during mitosis.
Stating that plant cells burst in a hypotonic solution. Plant cells become turgid, not burst. The cell wall prevents bursting. Only animal cells can burst (lyse).
Confusing resolution and magnification. Magnification makes the image bigger; resolution is the ability to distinguish between two close objects.

Practice Questions

Describe three differences between plant and animal cells.
Explain how a root hair cell is adapted for its function of absorbing water and mineral ions.
Describe the stages of the cell cycle, including what happens during interphase.
Explain the ethical issues surrounding the use of embryonic stem cells in medicine.
A red blood cell is placed in a very concentrated salt solution. Describe and explain what happens to the cell.
Calculate the actual size of a cell that measures 8.5 mm in a micrograph taken at a magnification of $\times 400$ . Give your answer in micrometres ( $\mu$ M).
Explain the difference between diffusion, osmosis, and active transport.
Describe how cancer develops and explain two risk factors.
Explain why cells need to divide by mitosis rather than growing larger.
A student sets up an osmosis experiment with potato chips. The results show that chips in 0.2 mol/dm $^3$ sugar solution increased in mass by 5%, while chips in 0.8 mol/dm $^3$ solution decreased in mass by 12%. Explain these results.
(Higher Tier) Explain why electron microscopes have a higher resolution than light microscopes, and describe one limitation of using electron microscopy.
(Higher Tier) A student observes cheek cells under a microscope at $\times 400$ magnification. The cells measure approximately 0.05 mm across the image. Calculate the actual size of the cells in micrometres.
Explain why it is important that the cells produced by mitosis are genetically identical to the parent cell. Use a specific example in your answer.
A scientist is investigating the effect of temperature on the rate of diffusion of a dye in water. Predict and explain how increasing the temperature from 20 $^{\circ}$ C to 40 $^{\circ}$ C would affect the rate of diffusion.
Describe the structure of a palisade mesophyll cell and explain how each feature is adapted for photosynthesis.
Explain why active transport is necessary in the small intestine for the absorption of glucose, even when there is already a higher concentration of glucose in the blood than in the gut.
Describe how you would use a light microscope to observe and measure the size of an onion epidermal cell. Include details of how you would calculate the actual size.
Explain the role of the spindle fibres during mitosis. What would happen if the spindle fibres did not function correctly?
A potato cylinder was placed in a 0.4 mol/dm $^3$ sugar solution and its mass decreased by 8%. Explain this result in terms of water potential.
Evaluate the use of stem cells in medicine. In your answer, consider both the potential benefits and the ethical concerns.
Describe the similarities and differences between ciliated epithelial cells and root hair cells in terms of their structure and how they are adapted to their functions.
Explain why bacteria can develop resistance to antibiotics much faster than humans can develop resistance to viral infections. Refer to both prokaryotic and eukaryotic cell biology in your answer.
A student investigating osmosis used potato cylinders that were 50 mm long at the start. After being placed in a 0.6 mol/dm $^3$ sugar solution for 30 minutes, the cylinders were 47 mm long. Calculate the percentage change in length and explain why measuring length is less reliable than measuring mass in osmosis experiments.
Compare the processes of mitosis and binary fission. In your answer, refer to the type of cell that undergoes each process, the number of daughter cells produced, and the genetic identity of the daughter cells.
Explain how the structure of the cell membrane (phospholipid bilayer) relates to its function as a partially permeable barrier. Why can small non-polar molecules cross more than large polar molecules?

Practice Problems

Question 1: Microscopy magnification

A cell is observed under a microscope with an eyepiece lens of magnification $\times 10$ and an Objective lens of magnification $\times 40$ . The cell appears to be $4.8 \mathrm{ mm$ wide in the Image. Calculate the actual width of the cell in micrometres ( $\mu$ M).

Answer

Total magnification $= 10 \times 40 = \times 400$ .

Actual size $= \frac{\mathrm{Image size}{\mathrm{Magnification} = \frac{4.8 \mathrm{ mm}{400} = 0.012 \mathrm{ mm$ .

Convert to micrometres: $0.012 \times 1000 = 12 \mathrm{ \mu m$ .

Question 2: Osmosis in plant cells

A plant cell with a solute potential of $-500 \mathrm{ kPa$ and a pressure potential of $200 \mathrm{ kPa$ is placed in a solution with a water potential of $-100 \mathrm{ kPa$ . Predict The direction of water movement and explain what will happen to the cell.

Answer

Cell water potential: $\psi = -500 + 200 = -300 \mathrm{ kPa$ .

The solution has $\psi = -100 \mathrm{ kPa$ .

Water moves from higher to lower water potential: from the solution ( $-100 \mathrm{ kPa$ ) into the Cell ( $-300 \mathrm{ kPa$ ).

The cell will gain water, increasing its pressure potential. The cell will become turgid. The cell Wall will prevent bursting.

Question 3: Prokaryotic vs eukaryotic cells

A student examines two cells under a microscope. Cell A has a nucleus, mitochondria, and a cell wall Made of cellulose. Cell B has no nucleus, no membrane-bound organelles, and a cell wall made of Peptidoglycan. Identify each cell type and explain two differences between them.

Answer

Cell A is a plant cell (eukaryotic). Cell B is a bacterial cell (prokaryotic).

Two differences: (1) Cell A has membrane-bound organelles (mitochondria); Cell B does not. (2) Cell A has a cell wall made of cellulose; Cell B has a cell wall made of peptidoglycan. (Other valid Differences: DNA structure, size, ribosome type, etc.)

Question 4: Enzyme activity and pH

An enzyme has an optimum pH of 7. A student tests the rate of reaction at pH 5, pH 7, and pH 9. Predict the relative rates and explain the shape of the resulting graph.

Answer

The rate is highest at pH 7 (the optimum). At pH 5 and pH 9, the rate is lower because changes in pH Alter the ionisation of amino acid side chains in the active site, disrupting the enzyme’s shape and Reducing substrate binding. The graph is a bell-shaped curve, symmetric if the enzyme tolerates acid And alkali equally, peaking at pH 7.

Question 5: Cell specialisation

Explain how a root hair cell is specialised for its function of absorbing water and mineral ions From the soil.

Answer

Root hair cells have an elongated, finger-like projection that greatly increases the surface area For absorption. They have a large number of mitochondria to provide ATP for active transport of Mineral ions against their concentration gradient. They have thin walls to reduce the diffusion Distance for water uptake. Their permanent vacuole contains a high concentration of solutes to Maintain a steep water potential gradient for osmosis.

Worked Examples

Example 1:

A typical exam question on Cell Biology requires you to apply your knowledge to an unfamiliar context. Read the question carefully, identify the key concept being tested, and structure your answer using the appropriate terminology.

Example 2:

Multi-step problems in Cell Biology often combine two or more concepts. Break the problem down: identify what you need to find, recall the relevant formula or principle, substitute values, and state your answer with correct units or formatting.

Summary

This topic covers the biological principles of cell biology, including key concepts, experimental evidence, and real-world applications.

Key concepts include:

cell structure (prokaryotic vs eukaryotic)
cell ultrastructure (organelles)
microscopy and resolution
cell division (mitosis and meiosis)
the cell cycle

Success requires the ability to recall specific factual content, apply knowledge to novel scenarios, and evaluate experimental evidence critically.

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